Highly flowable thermoplastic compositions are of interest for a wide variety of injection molding applications. By way of example, thin-walled components in the electrical, electronics and motor vehicle industry require low viscosities from the thermoplastics composition in order to permit material to be charged to the mold with minimum injection pressures and, respectively; clamping forces in the appropriate injection molding machines. This also applies to simultaneous charging of material to two or more injection molding components by way of a shared runner system in what are known as multicavity tooling systems. Shorter cycle times can moreover often be achieved using low-viscosity thermoplastic compositions. Good flowabilities are also specifically very important for highly filled thermoplastic compositions, e.g. with glass fibre and/or mineral contents above 40% by weight.
However, although the thermoplastic compositions have high flowability, the actual components produced therefrom are subjected to stringent mechanical requirements, and the lowering of viscosity cannot therefore be permitted to impair mechanical properties.
There are a number of ways of obtaining highly flowable, low-viscosity thermoplastic molding compositions.
One way uses low-viscosity polymer resins with very low molecular weight as base polymers for the thermoplastic molding compositions. However, the use of low-molecular-weight polymer resins is often associated with sacrifices in mechanical properties, in particular toughness. Preparation of a low-viscosity polymer resin in an existing polymerization plant moreover often requires complicated intervention attended by capital expenditure.
Another way uses what are known as flow aids, also termed flow agents or flow assistants or internal lubricants, which can be added as an additive to the polymer resin.
These flow aids are known from the literature, e.g. in Kunststoffe 2000, 9, p. 116-118, and by way of example can be fatty acid esters of polyols, or amides derived from fatty acids and from amines. However, these fatty acid esters, such as pentaerythritol tetrastearate or ethylene glycol dimontanoate, have only limited miscibility with polar thermoplastics, such as polyamides, polyalkylene terephthalates or polycarbonates. Their concentration increases at the surface of the molding and they are therefore also used as mold-release aids. However, on heat-ageing or else, in the case of polyamides, on absorption of moisture, particularly if concentrations are relatively high, the flow aids can migrate out of these moldings to the surface and become concentrated there. By way of example, in coated moldings this can lead to problems with regard to adhesion to paint or to metal.
As an alternative to the surface-active flow aids, it is possible to use internal flow aids which are compatible with the polymer resins. Examples of those suitable for this purpose are low-molecular-weight compounds or branched, highly branched or dendritic polymers whose polarity is similar to that of the polymer resin. These highly branched or dendritic systems are known from the literature and their basis can by way of example be branched polyesters, polyamides, polyesteramides, polyethers or polyamines, as described in Kunststoffe 2001, 91, pp. 179-190, or in Advances in Polymer Science 1999, 143 (Branched Polymers II), pp. 1-34.
EP-A 0 682 057 describes the use of the nitrogen-containing first-generation 4-cascade dendrimer: 1,4-diaminobutane[4]propylamine (N,N′-tetrabis(3-aminopropyl)-1,4-butanediamine) DAB(PA)4 to lower viscosity in nylon-6, nylon-6,6 and polybutylene terephthalate (PBT). While use of DAB(PA)4 to lower viscosity in polyamides has practically no effect on the impact resistance of the resultant molding compositions (difference <5%), impact resistance falls by more than 15% in the case of PBT.
WO-A 95/06081 (=U.S. Pat. No. 5,493,000) describes the use of three-dimensional branched polymers having rigid aromatic units in blends with polyamide in order to increase the stiffness of the material and the ultimate tensile strength, with simultaneous reduction in viscosity and in the tensile strain at break of the blends.
EP-A 0 994 157 (=AU 6 233 499 A) describes the use of highly branched polymers which are based on aromatics and which are added during caprolactam polycondensation and are therefore copolymerized. Compositions composed of polyamides in which highly branched polymers have been copolymerized exhibit better mechanical properties and better flowabilities here than comparative compositions without the highly branched components. Addition of the highly branched polymers during the polymerization reaction is described, but no addition to a polymer melt is described.
In principle, improvements in the flowability of polyamides can also be achieved via addition of phenols, of bisphenols, and of similar low-molecular-weight additives. EP-A 0 240 887 (=U.S. Pat. No. 5,212,224) describes molding compositions composed of polyamide, of a rubber and of a bisphenol, these exhibiting improved flowability brought about via the additive.
Alongside improvement in flowability, it is often desirable to improve the toughness of the materials. For this, other copolymers which are based on ethene and on acrylic or methacrylic esters and which bring about an improvement in toughness can also be added to the thermoplastics used.
DE-A 2 758 568 (=U.S. Pat. No. 4,362,846) and DE-A 2 801 585 (=U.S. Pat. No. 4,362,846) describe modification of the toughness of polyamides with acrylate-grafted polyolefins. It is emphasized that the use of the acrylate-modified polyolefins leads to an increase in melt viscosity.
EP-A 1 191 067 (=U.S. Pat. No. 6,759,480) describes modification of the toughness of thermoplastics, inter alia of polyamide and polybutylene terephthalate, via a mixture composed of a copolymer composed of ethene with an unreactive alkyl acrylate, and also of a copolymer composed of ethene with an acrylate having an additional reactive group. There is no discussion of the flowability of the molding compositions.
FR-A 2 819 821 describes the use of copolymers composed of ethene with 2-ethylhexyl acrylate whose MFI (melt flow index) is smaller than 100 as a constituent of hot-melt adhesive mixtures. There are no indications of applications for elastomer modification and/or flowability improvement in semicrystalline thermoplastics.